Increased production of isobutanol in yeast with reduced mitochondrial amino acid biosynthesis

ABSTRACT

Yeast cells with reduced activity of certain enzymes involved in branched chain amino acid biosynthesis in yeast mitochondria are described. Target enzymes include threonine deaminase, isopropylmalate synthase, and optionally branched chain amino acid transaminase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of U.S. application Ser. No. 14/302,097, filed Jun. 11, 2014, which is a divisional and claims the benefit of U.S. application Ser. No. 13/889,999, filed on May 8, 2013, now U.S. Pat. No. 8,785,166, which is a divisional of and claims the benefit of U.S. application Ser. No. 12/617,039, filed Nov. 12, 2009, now U.S. Pat. No. 8,465,964, which is related to and claims the benefit of priority of U.S. Provisional Application No. 61/114,072, filed Nov. 13, 2008. Each of the referenced applications is herein incorporated by reference.

FIELD OF THE INVENTION

The invention relates to the field of microbiology. More specifically, recombinant yeast strains are disclosed that have reduced amino acid biosynthesis and increased isobutanol production.

BACKGROUND OF THE INVENTION

Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase.

Isobutanol is produced biologically as a by-product of yeast fermentation. It is a component of “fusel oil” that forms as a result of incomplete metabolism of amino acids by this group of fungi. Isobutanol is specifically produced from catabolism of L-valine in the cytoplasm. After the amine group of L-valine is harvested as a nitrogen source, the resulting α-keto acid is dicarboxylate and reduced to isobutanol by enzymes of the so-called Enrich pathway (Dickinson et al., J. Biol. Chem. 273(40):25752-25756 (1998)). Yields of fusel oil and/or its components achieved during beverage fermentation are typically low. For example, the concentration of isobutanol produced in beer fermentation is reported to be less than 16 parts per million (Garcia et al., Process Biochemistry 29:303-309 (1994)). Addition of exogenous L-valine to the fermentation increases the yield of isobutanol, as described by Dickinson et al., supra, wherein it is reported that a yield of isobutanol of 3 g/L is obtained by providing L-valine at a concentration of 20 g/L in the fermentation. However, the use of valine as a feed-stock would be cost prohibitive for industrial scale isobutanol production.

Additionally Van Nedervelde et al (Proceedings of the Congress—European Brewery Convention (2003), 29th, 50/1-50/10) have demonstrated the deletions in the gene encoding the BAT 1 mitochondrial protein in yeast result in strains having increased levels of higher alcohols. Similarly Nako et al (WO 2007032522) note that amyl alcohol and/or isobutanol and/or isoamyl acetate levels in yeast used for the production of alcoholic beverages may be altered via manipulation of the BAT1 and BAT2 genes. The art is silent with respect to the down regulation of other genes encoding proteins that are functional in the mitochondria for the enhanced production of isobutanol in yeast.

There is a need for attaining higher amounts of isobutanol through yeast fermentation without addition of valine or other isobutanol production intermediates.

SUMMARY OF THE INVENTION

Provided herein are recombinant yeast host cells which comprise mitochondria which are substantially devoid of an enzyme activity selected from the group consisting of threonine deaminase and isopropylmalate synthase activity. In some embodiments, the host cells produce isobutanol. In some embodiments, the mitochondria is substantially devoid of branched chain amino acid transaminase activity, and in some embodiments, the mitochondria is substantially devoid of pyruvate dehydrogenase activity. In some embodiments, endogenous pyruvate decarboxylase activity is reduced.

In some embodiments, the threonine deaminase activity is defined by the enzyme classification number EC 4.3.1.19 and the isopropylmalate synthase activity is defined by the enzyme classification number EC 2.3.3.13. In some embodiments, the branched chain amino acid transaminase activity is defined by the enzyme classification number EC 2.6.1.42. In some embodiments, the pyruvate dehydrogenase activity is defined by the enzyme classification number EC 1.2.4.1. In some embodiments, the pyruvate dehydrogenase activity is defined by a multienzyme complex comprising proteins selected from the group consisting of: PDA1, PDA1, PDB1, LAT1, LPD1, and PDX1.

In some embodiments, the yeast is selected from the group consisting of Saccharomyces, Schizosaccharomyces, Hansenula, Candida, Kluyveromyces, Yarrowia and Pichia.

In some embodiments, yeast cells provided comprise a disruption in a gene selected from the group consisting of ILV1 and LEU4. In some embodiments, yeast cells provided comprise a disruption in the BAT1 gene. In some embodiments, yeast cells provided comprise a disruption in a gene encoding a protein selected from the group consisting of PDA1, PDA1, PDB1, LAT1, LPD1, and PDX1

In some embodiments, yeast cells provided herein are Saccharomyces and wherein; a) the ILV1 gene encodes a polypeptide having at least 80% identity to an amino acid sequence as set forth in SEQ ID NO: 2, based on the Clustal W method of alignment using the default parameters of GAP PENALTY=10, GAP LENGTH PENALTY=0.1, and Gonnet 250 series of protein weight matrix; and b) the LEU4 gene encodes a polypeptide having at least 80% identity to the amino acid sequence set forth in SEQ ID NO: 28 based on the Clustal W method of alignment using the default parameters of GAP PENALTY=10, GAP LENGTH PENALTY=0.1, and Gonnet 250 series of protein weight matrix.

In some embodiments, yeast cells provided herein are Saccharomyces and wherein the BAT1 gene encodes a protein having at least 80% identity to the amino acid sequence as set forth in SEQ ID NO:16 based on the Clustal W method of alignment using the default parameters of GAP PENALTY=10, GAP LENGTH PENALTY=0.1, and Gonnet 250 series of protein weight matrix.

In some embodiments, yeast cells provided herein are Saccharomyces and wherein; a) the PDA1 polypeptide has at least 80% identity to the amino acid sequence as set forth in SEQ ID NO:70; b) the PDB1 polypeptide has at least 80% identity to the amino acid sequence as set forth in SEQ ID NO: 58; c) the LAT1 polypeptide has at least 80% identity to the amino acid sequence as set forth in SEQ ID NO: 106; d) the LPD1 polypeptide has at least 80% identity to the amino acid sequence as set forth in SEQ ID NO: 108; and e) the PDX1 polypeptide has at least 80% identity to the amino acid sequence as set forth in SEQ ID NO: 110; wherein identity of polypeptides recited in parts (a)-(e) is based on the Clustal W method of alignment using the default parameters of GAP PENALTY=10, GAP LENGTH PENALTY=0.1, and Gonnet 250 series of protein weight matrix.

Also provided herein are methods for the production of isobutanol comprising growing provided host cells under conditions wherein isobutanol is produced.

BRIEF DESCRIPTION OF THE FIGURES AND SEQUENCE DESCRIPTIONS

The invention can be more fully understood from the following detailed description, figures, and the accompanying sequence descriptions, which form a part of this application.

FIG. 1 shows biosynthetic pathways for amino acids in yeast mitochondria.

The following sequences conform with 37 C.F.R. 1.821-1.825 (“Requirements for Patent Applications Containing Nucleotide Sequences and/or Amino Acid Sequence Disclosures—the Sequence Rules”) and are consistent with World Intellectual Property Organization (WIPO) Standard ST.25 (1998) and the sequence listing requirements of the EPO and PCT (Rules 5.2 and 49.5(a-bis), and Section 208 and Annex C of the Administrative Instructions). The symbols and format used for nucleotide and amino acid sequence data comply with the rules set forth in 37 C.F.R. §1.822.

TABLE 1 SEQ ID NOs of target proteins and encoding sequences for reduction SEQ ID NO: SEQ ID NO: Organism and gene name nucleic acid amino acid Saccharomyces cerevisiae YJM789, Ilv1 1 2 Schizosaccharomyces pombe, Ilv1 3 4 Candida albicans SC5314, Ilv1 5 6 Candida glabrata, Ilv1 7 8 Kluyveromyces lactis, Ilv1 9 10 Yarrowia lipolytica strain CLIB122, Ilv1 11 12 Pichia stipitis CBS 6054, Ilv1 13 14 Saccharomyces cerevisiae, BAT1 15 16 Schizosaccharomycs pombe, BAT1 17 18 Candida albicans SC5314, BAT1 19 20 Kluyveromyces lactis, BAT1 21 22 Yarrowia lipolytica, BAT1 23 24 Pichia stipitis CBS 6054, BAT1 25 26 Saccharomyces cerevisiae, Leu4 27 28 Schizosaccharomycs pombe, Leu4 29 30 chromosome II Schizosaccharomycs pombe, 31 32 Leu4, NP_596103.2 Candida albicans SC5314, Leu4 33 34 Candida albicans SC5314, Leu4 35 36 Candida albicans SC5314, Leu4 37 38 Candida albicans SC5314, Leu4 39 40 Candida glabrata, Leu4; XP_446653.1 41 42 Candida glabrata, Leu4; XP_446566.1 43 44 Kluyveromyces lactis, Leu4; CAH00792.1 45 46 Kluyveromyces lactis, Leu4; CAG98836.1 47 48 Yarrowia lipolytica, Leu4, CAA88928.1 49 50 Yarrowia lipolytica, Leu4 51 52 Pichia stipitis CBS 6054, Leu4, 53 54 XP_001387341.1 Pichia stipitis CBS 6054, Leu4, 55 56 XP_001384536.2 Saccharomyces cerevisiae, PDB1 57 58 Schizosaccharomycs pombe, PDB1 59 60 Candida albicans SC5314, PDB1 61 62 Kluyveromyces lactis, PDB1 63 64 Yarrowia lipolytica, PDB1 65 66 Pichia stipitis CBS 6054, PDB1 67 68 Saccharomyces cerevisiae, PDA1 69 70 Schizosaccharomycs pombe, PDA1 71 72 Candida albicans, PDA1 73 74 Kluyveromyces lactis, PDA1 75 76 Yarrowia lipolytica, PDA1 hypothetical 77 78 protein Pichia stipitis, PDA1 79 80 Saccharomyces cerevisiae Lat1 pyruvate 105 106 dehydrogenase complex Saccharomyces cerevisiae Lpd1 107 108 pyruvate dehydrogenase complex Saccharomyces cerevisiae Pdx1 pyruvate 109 110 dehydrogenase complex

TABLE 2 SEQ ID NOs for primers and vectors Primer or vector name Description SEQ ID NO 112590-88A Primer 81 112590-88B Primer 82 112590-88C Primer 83 112590-88D Primer 84 pUC19-URA3r Vector 85 112590-97A Primer 86 112590-97B Primer 87 112590-49E Primer 88 112590-97C Primer 89 112590-108A Primer 90 112590-108B Primer 91 112590-108C Primer 92 112590-108D Primer 93 112590-108E Primer 94 112590-108F Primer 95 BAT1 check Primer 96 112590-118A Primer 97 112590-118B Primer 98 pRS426::GAL1p-alsS Vector 99 112590-118C Primer 100 112590-118D Primer 101 112590-118E Primer 102 112590-118F Primer 103 112590-118G Primer 104

TABLE 3 Yeast pyruvate decarboxylase sequences SEQ ID NO: SEQ ID NO: Organism and gene name nucleic acid amino acid PDC1 pyruvate decarboxylase 108 109 from Saccharomyces cerevisiae PDC5 pyruvate decarboxylase 110 111 from Saccharomyces cerevisiae PDC6 pyruvate decarboxylase 112 113 from Saccharomyces cerevisiae Pyruvate decarboxylase from 114 115 Candida glabrata PDC1 pyruvate decarboxylase 116 117 from Pichia stipitis PDC2 pyruvate decarboxylase 118 119 from Pichia stipitis Pyruvate decarboxylase from 120 121 Kluyveromyces lactis Pyruvate decarboxylase from 122 123 Yarrowia lipolytica Pyruvate decarboxylase from 124 125 Schizosaccharomyces pombe

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to recombinant yeast cells engineered to have reduced activity of threonine deaminase and isopropylmalate synthase, and optionally reduced activity of branched chain amino acid transaminase, in the mitochondria. These cells produce increased amounts of isobutanol as compared to cells with normal levels of these enzyme activities. Isobutanol is valuable as a fuel or fuel additive to reduce demand for fossil fuels.

The following abbreviations and definitions will be used for the interpretation of the specification and the claims.

As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, a mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

Also, the indefinite articles “a” and “an” preceding an element or component of the invention are intended to be nonrestrictive regarding the number of instances (i.e. occurrences) of the element or component. Therefore “a” or “an” should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular.

The term “invention” or “present invention” as used herein is a non-limiting term and is not intended to refer to any single embodiment of the particular invention but encompasses all possible embodiments as described in the specification and the claims.

As used herein, the term “about” modifying the quantity of an ingredient or reactant of the invention employed refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or use solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients employed to make the compositions or carry out the methods; and the like. The term “about” also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or not modified by the term “about”, the claims include equivalents to the quantities. In one embodiment, the term “about” means within 10% of the reported numerical value, preferably within 5% of the reported numerical value.

The term “isobutanol biosynthetic pathway” refers to an enzyme pathway to produce isobutanol from pyruvate.

The term “threonine deaminase refers to an enzyme having the EC number EC 4.3.1.19 (Enzyme Nomenclature 1992, Academic Press, San Diego). Threonine deaminase catalyzes the reaction of threonine to 2-oxobutanoate. This is an enzyme involved in branched chain amino acid biosynthesis, specifically of isoleucine. The gene encoding this enzyme is localized in the yeast nucleus however the encoded protein is transported to the mitochondria where it is metabolically active.

The term “isopropylmalate synthase” refers to an enzyme having the EC number EC 2.3.3.13 (Enzyme Nomenclature 1992, Academic Press, San Diego). Isopropylmalate synthase catalyzes the reaction of alpha-ketoisovalerate to isopropyl malate. This is an enzyme involved in branched chain amino acid biosynthesis, specifically of leucine. The gene encoding this enzyme is localized in the yeast nucleus however the encoded protein is transported to the mitochondria where it is metabolically active.

The term “branched chain amino acid transaminase” refers to an enzyme having the EC number EC 2.6.1.42 (Enzyme Nomenclature 1992, Academic Press, San Diego). Branched chain amino acid transaminase catalyzes the reaction of alpha-ketoisovalerate to valine and catalyzes the reaction of 2-keto-3-methyl-valerate to isoleucine. This is an enzyme involved in branched chain amino acid biosynthesis, specifically of valine and isoleucine. The gene encoding this enzyme is localized in the yeast nucleus however the encoded protein is transported to the mitochondria where it is metabolically active.

The term “pyruvate dehydrogenase” refers to an activity provided by a multienzyme complex that may include proteins PDA1, PDB1, LAT1, LPD1, and PDX1. PDA1 and PDB1 are E1α and E1β subunits of pyruvate dehydrogenase which has EC number EC 1.2.4.1. LAT1 is dihydrolipoyllysine-residue acetyltransferase, also called dihydrolopoyl transacetylase, which has EC number EC 2.3.1.12. LPD1 is dihydrolipoyl dehydrogenase which has EC number EC 1.8.1.4. Pyruvate dehydrogenase activity catalyzes the reaction of pyruvate to acetyl-CoA. The gene encoding this enzyme is localized in the yeast nucleus however the encoded protein is transported to the mitochondria where it is metabolically active.

The term “carbon substrate” or “fermentable carbon substrate” refers to a carbon source capable of being metabolized by host organisms of the present invention and particularly carbon sources selected from the group consisting of monosaccharides, oligosaccharides, polysaccharides, and one-carbon substrates or mixtures thereof.

The term “gene” refers to a nucleic acid fragment that is capable of being expressed as a specific protein, optionally including regulatory sequences preceding (5′ non-coding sequences) and following (3′ non-coding sequences) the coding sequence. “Native gene” refers to a gene as found in nature with its own regulatory sequences. “Chimeric gene” refers to any gene that is not a native gene, comprising regulatory and coding sequences that are not found together in nature. Accordingly, a chimeric gene may comprise regulatory sequences and coding sequences that are derived from different sources, or regulatory sequences and coding sequences derived from the same source, but arranged in a manner different than that found in nature. “Endogenous gene” refers to a native gene in its natural location in the genome of an organism. A “foreign gene” or “heterologous gene” refers to a gene not normally found in the host organism, but that is introduced into the host organism by gene transfer. “Heterologous gene” includes a native coding region, or portion thereof, that is reintroduced into the source organism in a form that is different from the corresponding native gene. For example, a heterologous gene may include a native coding region that is a portion of a chimeric gene including non-native regulatory regions that is reintroduced into the native host. Also foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A “transgene” is a gene that has been introduced into the genome by a transformation procedure.

As used herein the term “coding region” refers to a DNA sequence that codes for a specific amino acid sequence. “Suitable regulatory sequences” refer to nucleotide sequences located upstream (5′ non-coding sequences), within, or downstream (3′ non-coding sequences) of a coding sequence, and which influence the transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences may include promoters, translation leader sequences, introns, polyadenylation recognition sequences, RNA processing site, effector binding site and stem-loop structure.

The term “promoter” refers to a DNA sequence capable of controlling the expression of a coding sequence or functional RNA. In general, a coding sequence is located 3′ to a promoter sequence. Promoters may be derived in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters may direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental or physiological conditions. Promoters which cause a gene to be expressed in most cell types at most times are commonly referred to as “constitutive promoters”. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths may have identical promoter activity.

The term “operably linked” refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably linked with a coding sequence when it is capable of effecting the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in sense or antisense orientation.

The term “expression”, as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragment of the invention. Expression may also refer to translation of mRNA into a polypeptide.

The term “substantially devoid” when used in reference to the presence of an enzyme activity in a host cell means that the presences of that enzyme is not detectable using conventional assay methods or is detectable at such low levels that the presence of the enzyme at would not be expected to have any effect on metabolic pathways.

As used herein the term “transformation” refers to the transfer of a nucleic acid fragment into a host organism, resulting in genetically stable inheritance. Host organisms containing the transformed nucleic acid fragments are referred to as “transgenic” or “recombinant” or “transformed” organisms.

The terms “plasmid” and “vector” as used herein, refer to an extra chromosomal element often carrying genes which are not part of the central metabolism of the cell, and usually in the form of circular double-stranded DNA molecules. Such elements may be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear or circular, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3′ untranslated sequence into a cell.

As used herein the term “codon degeneracy” refers to the nature in the genetic code permitting variation of the nucleotide sequence without effecting the amino acid sequence of an encoded polypeptide. The skilled artisan is well aware of the “codon-bias” exhibited by a specific host cell in usage of nucleotide codons to specify a given amino acid. Therefore, when synthesizing a gene for improved expression in a host cell, it is desirable to design the gene such that its frequency of codon usage approaches the frequency of preferred codon usage of the host cell.

As used herein, an “isolated nucleic acid fragment” or “isolated nucleic acid molecule” will be used interchangeably and will mean a polymer of RNA or DNA that is single- or double-stranded, optionally containing synthetic, non-natural or altered nucleotide bases. An isolated nucleic acid fragment in the form of a polymer of DNA may be comprised of one or more segments of cDNA, genomic DNA or synthetic DNA.

A nucleic acid fragment is “hybridizable” to another nucleic acid fragment, such as a cDNA, genomic DNA, or RNA molecule, when a single-stranded form of the nucleic acid fragment can anneal to the other nucleic acid fragment under the appropriate conditions of temperature and solution ionic strength. Hybridization and washing conditions are well known and exemplified in Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual, 2^(nd) ed., Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989), particularly Chapter 11 and Table 11.1 therein (entirely incorporated herein by reference). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. Stringency conditions can be adjusted to screen for moderately similar fragments (such as homologous sequences from distantly related organisms), to highly similar fragments (such as genes that duplicate functional enzymes from closely related organisms). Post-hybridization washes determine stringency conditions. One set of preferred conditions uses a series of washes starting with 6×SSC, 0.5% SDS at room temperature for 15 min, then repeated with 2×SSC, 0.5% SDS at 45° C. for 30 min, and then repeated twice with 0.2×SSC, 0.5% SDS at 50° C. for 30 min. A more preferred set of stringent conditions uses higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2×SSC, 0.5% SDS was increased to 60° C. Another preferred set of highly stringent conditions uses two final washes in 0.1×SSC, 0.1% SDS at 65° C. An additional set of stringent conditions include hybridization at 0.1×SSC, 0.1% SDS, 65° C. and washes with 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1% SDS, for example.

Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of Tm for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher Tm) of nucleic acid hybridizations decreases in the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating Tm have been derived (see Sambrook et al., supra, 9.50-9.51). For hybridizations with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook et al., supra, 11.7-11.8). In one embodiment the length for a hybridizable nucleic acid is at least about 10 nucleotides. Preferably a minimum length for a hybridizable nucleic acid is at least about 15 nucleotides; more preferably at least about 20 nucleotides; and most preferably the length is at least about 30 nucleotides. Furthermore, the skilled artisan will recognize that the temperature and wash solution salt concentration may be adjusted as necessary according to factors such as length of the probe.

A “substantial portion” of an amino acid or nucleotide sequence is that portion comprising enough of the amino acid sequence of a polypeptide or the nucleotide sequence of a gene to putatively identify that polypeptide or gene, either by manual evaluation of the sequence by one skilled in the art, or by computer-automated sequence comparison and identification using algorithms such as BLAST (Altschul, S. F., et al., J. Mol. Boil., 215:403-410 (1993)). In general, a sequence of ten or more contiguous amino acids or thirty or more nucleotides is necessary in order to putatively identify a polypeptide or nucleic acid sequence as homologous to a known protein or gene. Moreover, with respect to nucleotide sequences, gene specific oligonucleotide probes comprising 20-30 contiguous nucleotides may be used in sequence-dependent methods of gene identification (e.g., Southern hybridization) and isolation (e.g., in situ hybridization of bacterial colonies or bacteriophage plaques). In addition, short oligonucleotides of 12-15 bases may be used as amplification primers in PCR in order to obtain a particular nucleic acid fragment comprising the primers. Accordingly, a “substantial portion” of a nucleotide sequence comprises enough of the sequence to specifically identify and/or isolate a nucleic acid fragment comprising the sequence. The instant specification teaches the complete amino acid and nucleotide sequence encoding particular fungal proteins. The skilled artisan, having the benefit of the sequences as reported herein, may now use all or a substantial portion of the disclosed sequences for purposes known to those skilled in this art. Accordingly, the instant invention comprises the complete sequences as reported in the accompanying Sequence Listing, as well as substantial portions of those sequences as defined above.

The term “complementary” is used to describe the relationship between nucleotide bases that are capable of hybridizing to one another. For example, with respect to DNA, adenosine is complementary to thymine and cytosine is complementary to guanine.

The term “percent identity”, as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences. In the art, “identity” also means the degree of sequence relatedness between polypeptide or polynucleotide sequences, as the case may be, as determined by the match between strings of such sequences. “Identity” and “similarity” can be readily calculated by known methods, including but not limited to those described in: 1.) Computational Molecular Biology (Lesk, A. M., Ed.) Oxford University: NY (1988); 2.) Biocomputing: Informatics and Genome Protects (Smith, D. W., Ed.) Academic: NY (1993); 3.) Computer Analysis of Sequence Data, Part I (Griffin, A. M., and Griffin, H. G., Eds.) Humania: NJ (1994); 4.) Sequence Analysis in Molecular Biology (von Heinje, G., Ed.) Academic (1987); and 5.) Sequence Analysis Primer (Gribskov, M. and Devereux, J., Eds.) Stockton: NY (1991).

Preferred methods to determine identity are designed to give the best match between the sequences tested. Methods to determine identity and similarity are codified in publicly available computer programs. Sequence alignments and percent identity calculations may be performed using the MegAlign™ program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignment of the sequences is performed using the “Clustal method of alignment” which encompasses several varieties of the algorithm including the “Clustal V method of alignment” corresponding to the alignment method labeled Clustal V (described by Higgins and Sharp, CABIOS. 5:151-153 (1989); Higgins, D. G. et al., Comput. Appl. Biosci., 8:189-191 (1992)) and found in the MegAlign™ program of the LASERGENE bioinformatics computing suite (DNASTAR Inc.). For multiple alignments, the default values correspond to GAP PENALTY=10 and GAP LENGTH PENALTY=10. Default parameters for pairwise alignments and calculation of percent identity of protein sequences using the Clustal method are KTUPLE=1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5. For nucleic acids these parameters are KTUPLE=2, GAP PENALTY=5, WINDOW=4 and DIAGONALS SAVED=4. After alignment of the sequences using the Clustal V program, it is possible to obtain a “percent identity” by viewing the “sequence distances” table in the same program. Additionally the “Clustal W method of alignment” is available and corresponds to the alignment method labeled Clustal W (described by Higgins and Sharp, CABIOS. 5:151-153 (1989); Higgins, D. G. et al., Comput. Appl. Biosci. 8:189-191 (1992)) and found in the MegAlign™ v6.1 program of the LASERGENE bioinformatics computing suite (DNASTAR Inc.). Default parameters for multiple alignment (GAP PENALTY=10, GAP LENGTH PENALTY=0.2, Delay Divergen Seqs (%)=30, DNA Transition Weight=0.5, Protein Weight Matrix=Gonnet Series, DNA Weight Matrix=IUB). After alignment of the sequences using the Clustal W program, it is possible to obtain a “percent identity” by viewing the “sequence distances” table in the same program.

It is well understood by one skilled in the art that many levels of sequence identity are useful in identifying polypeptides, from other species, wherein such polypeptides have the same or similar function or activity. Useful examples of percent identities include, but are not limited to: 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%, or any integer percentage from 55% to 100% may be useful in describing the present invention, such as 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%. Suitable nucleic acid fragments not only have the above homologies but typically encode a polypeptide having at least 50 amino acids, preferably at least 100 amino acids, more preferably at least 150 amino acids, still more preferably at least 200 amino acids, and most preferably at least 250 amino acids.

The term “sequence analysis software” refers to any computer algorithm or software program that is useful for the analysis of nucleotide or amino acid sequences. “Sequence analysis software” may be commercially available or independently developed. Typical sequence analysis software will include, but is not limited to: 1.) the GCG suite of programs (Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison, Wis.); 2.) BLASTP, BLASTN, BLASTX (Altschul et al., J. Mol. Biol., 215:403-410 (1990)); 3.) DNASTAR (DNASTAR, Inc. Madison, Wis.); 4.) Sequencher (Gene Codes Corporation, Ann Arbor, Mich.); and 5.) the FASTA program incorporating the Smith-Waterman algorithm (W. R. Pearson, Comput. Methods Genome Res., [Proc. Int. Symp.] (1994), Meeting Date 1992, 111-20. Editor(s): Suhai, Sandor. Plenum: New York, N.Y.). Within the context of this application it will be understood that where sequence analysis software is used for analysis, that the results of the analysis will be based on the “default values” of the program referenced, unless otherwise specified. As used herein “default values” will mean any set of values or parameters that originally load with the software when first initialized.

Standard recombinant DNA and molecular cloning techniques used here are well known in the art and are described by Sambrook, J., Fritsch, E. F. and Maniatis, T., Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1989) (hereinafter “Maniatis”); and by Silhavy, T. J., Bennan, M. L. and Enquist, L. W., Experiments with Gene Fusions, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1984); and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, published by Greene Publishing Assoc. and Wiley-Interscience (1987). Additional methods used here are in Methods in Enzymology, Volume 194, Guide to Yeast Genetics and Molecular and Cell Biology (Part A, 2004, Christine Guthrie and Gerald R. Fink (Eds.), Elsevier Academic Press, San Diego, Calif.).

Mitochondrial Enzyme Engineering for Isobutanol Production

Disclosed herein are yeast cells with improved isobutanol production and engineering of endogenous enzyme activities in the mitochondria of yeast cells. This engineering may be performed in any type of yeast cell that is amenable to genetic engineering methods and that naturally produces at least a small amount of isobutanol as a by-product of incomplete amino acid metabolism. Suitable yeasts include, but are not limited to, Saccharomyces, Schizosaccharomyces, Hansenula, Candida, Kluyveromyces, Yarrowia and Pichia. Suitable strains include, but are not limited to, Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces thermotolerans, Candida glabrata, Candida albicans, Pichia stipitis and Yarrowia lipolytica.

Applicants have found that altering branched chain amino acid biosynthesis and pyruvate metabolism in the mitochondria of yeast can affect the amount of isobutanol produced by the yeast cell. Surprisingly, Applicants found that altered cells had increased isobutanol production using endogenous isobutanol biosynthesis. Applicants found that by eliminating threonine deaminase and isopropylmalate synthase activities in the yeast mitochondria, a nine-fold increase in isobutanol production was achieved. With further elimination of branched chain amino acid aminotransferase activity in the mitochondria, over twelve-fold increase in isobutanol production was achieved. Additional reduction of pyruvate dehydrogenase activity in the mitochondria resulted in over thirteen-fold increase in isobutanol production.

Mitochondrial biosynthetic pathways for branched chain amino acid biosynthesis are shown in the diagram in FIG. 1. The following enzymes are encoded by the genes labeled as steps (arrows) in the pathways in FIG. 1:

ILV1: threonine deaminase

ILV2: acetolactate synthase (ALS)

ILV3: dihydroxy-acid dehydratase (DHAD)

ILV5: acetohydroxy acid reductoisomerase (KARI)

BAT1: branched chain amino acid aminotransferase

BAT2: branched chain amino acid transaminase

LEU4: isopropylmalate synthase

PDA1*: refers to the complex including the components:

-   -   PDA1: pyruvate dehydrogenase E1α subunit     -   PDB1: pyruvate dehydrogenase E1β subunit     -   LAT1: dihydrolipoyllysine-residue acetyltransferase     -   LPD1: dihydrolipoyl dehydrogenase     -   PDX1: protein X         Threonine deaminase, ALS, KARI, DHAD and branched chain amino         acid aminotransferase enzyme activities in the mitochondria form         a biosynthetic pathway from threonine to isoleucine. ALS, KARI,         DHAD and branched chain amino acid aminotransferase enzyme         activities in the mitochondria form a biosynthetic pathway from         pyruvate to valine. ALS, KARI, DHAD and isopropylmalate synthase         enzyme activities in the mitochondria form a biosynthetic         pathway from pyruvate to isopropyl malate, which moves to the         cytoplasm and is converted to leucine.

While not intending to suggest a mechanism of the effect of down-regulating or eliminating the activity of various mitochondrial enzymes the presence of the cytosolic valine to isobutanol pathway generally in yeast suggests some possible explanations for the effects seen here. For example, eliminating threonine deaminase activity may affect pathway intermediate flow in the pathway from threonine to isoleucine. Eliminating isopropylmalate synthase activity may reduce metabolism of the α-ketoisovalerate intermediate in the leucine pathway. Eliminating mitochondrial branched chain amino acid aminotransferase activity may reduce metabolism of the alpha-ketoisovalerate intermediate in the valine pathway that is fully within the mitochondrion. Applicants found that the combination of the loss of threonine deaminase and isopropylmalate synthase activities in the mitochondria was very effective in increasing isobutanol production, suggesting that there was substantial increase in transfer of α-ketoisovalerate to the cytoplasm, and it was converted to isobutanol. In the present cells the conversion to isobutanol relies on endogenous enzyme activities of the yeast cell. Applicants found that the combination of the loss of threonine deaminase, isopropylmalate synthase, and branched chain amino acid aminotransferase activities in the mitochondria further increased isobutanol production, suggesting that there was further increase in transfer of α-ketoisovalerate to the cytoplasm, and it was converted to isobutanol.

In the yeast mitochondria pyruvate is also converted to acetyl-CoA through pyruvate dehydrogenase activity (see FIG. 1). Applicants found that eliminating pyruvate dehydrogenase activity in the mitochondria further increased isobutanol production, suggesting that flow of pyruvate to alpha-ketoisovalerate was increased, as well as transfer of alpha-ketoisovalerate to the cytoplasm with conversion to valine and then to isobutanol. The conversion of pyruvate to acetyl-CoA is catalyzed by a multienzyme pyruvate dehydrogenase complex. The pyruvate dehydrogenase enzyme is one enzyme of the multienzyme pyruvate dehydrogenase complex. Pyruvate dehydrogenase (EC 1.2.4.1) itself has alpha and beta subunits: PDA1 and PDB1, respectively, forming the E1α and E1β subunits, respectively, of the E1 component. The complex includes an E2 core which has dihydrolipoamide acetyltransferase activity (EC 2.3.1.12) and E3 which has dihydrolipoamide dehydrogenase activity (EC1.8.1.4). E2 may be encoded by LAT1 and E3 by LPD1. An additional complex protein is encoded by PDX1, which links Lat1p to Lpd1p. Thus the pyruvate dehydrogenase complex may include PDA1, PDB1, Lat1, Lpd1, and Pdx1, or homologous proteins encoded by genes which may have alternative names in various yeasts. The activity of any of these proteins may be reduced to affect the function of the pyruvate dehydrogenase complex, and thereby affect pyruvate dehydrogenase activity, to prepare a strain of one embodiment of the present invention. In the description below when referring to PDA1, it is understood that PDA1 may be substituted by any of PDB1, LAT1, LPD1, or PDX1, any of which may be modified to reduce pyruvate dehydrogenase activity.

In the present invention, any yeast enzymes providing threonine deaminase, isopropylmalate synthase, branched chain amino acid aminotransferase, or pyruvate dehydrogenase activities in the mitochondria may be targets for engineering to reduce these activities. Preferably the activity is reduced such that there is substantially no detectable activity of the target enzyme. Yeast cells are engineered to reduce enzyme activity typically by modification of the gene encoding the target enzyme. The genes encoding these enzymes are ILV1, LEU4, BAT1, and PDA1 (and multienzyme complex genes PDB1, LAT1, LPD1, and PDX1), respectively. Any ILV1, LEU4, BAT1, or PDA1 gene of yeast encoding a mitochondrial targeted protein is a target for engineering for reduced expression of the encoded enzyme activity in the present cells. Examples of target coding region sequences and their encoded proteins from different species of yeast cells are given as SEQ ID NOs: 1-80 and 105-110 in Table 1. Other target proteins, or their encoding sequences, having at least about 70%-75%, 75%-80%, 80-85%, 85%-90%, 90%-95%, or at least about 98% sequence identity to any of the proteins or coding sequences listed in Table 1, and these activities, may be identified in the literature and in bioinformatics databases well known to the skilled person.

There is cytoplasmic isopropylmalate synthase activity encoded by LEU9 and cytoplasmic branched chain amino acid transaminase activity encoded by BAT2, genes which are not targets in the present disclosure.

Because mitochondrial threonine deaminase, isopropylmalate synthase, branched chain amino acid aminotransferase, and pyruvate dehydrogenase complex enzymes are well known, as well as their encoding genes (ILV1, LEU4, BAT1, PDA1, PDB1, LAT1, LPD1, and PDX1, respectively), one skilled in the art can readily identify these proteins and their encoding genes in yeast cells using bioinformatics approaches, to identify additional target genes for engineering as disclosed herein. Typically BLAST (described above) searching of publicly available databases with known target protein sequences, such as those provided herein, is used to identify homologous proteins and their encoding sequences that may be targeted for inactivation in the present strains. For example, endogenous yeast mitochondrial threonine deaminase proteins having amino acid sequence identities of at least about 70%-75%, 75%-80%, 80-85%, 85%-90%, 90%-95% or 98% sequence identity to any of the threonine deaminase proteins of SEQ ID NOs:2, 4, 6, 8, 10, 12, and 14 may have reduced expression in the present strains. Identities are based on the Clustal W method of alignment using the default parameters of GAP PENALTY=10, GAP LENGTH PENALTY=0.1, and Gonnet 250 series of protein weight matrix.

In the following description, ILV1 is used as an example, and the same description applies to any of LEU4, BAT1, PDA1, PDB1, LAT1, LPD1, and PDX1 coding regions. The sequences of, for example, the ILV1 coding regions provided herein may be used to identify other homologs in nature. For example each of the threonine deaminase encoding nucleic acid fragments described herein may be used to isolate genes encoding homologous proteins. Isolation of homologous genes using sequence-dependent protocols is well known in the art. Examples of sequence-dependent protocols include, but are not limited to: 1.) methods of nucleic acid hybridization; 2.) methods of DNA and RNA amplification, as exemplified by various uses of nucleic acid amplification technologies [e.g., polymerase chain reaction (PCR), Mullis et al., U.S. Pat. No. 4,683,202; ligase chain reaction (LCR), Tabor, S. et al., Proc. Acad. Sci. USA 82:1074 (1985); or strand displacement amplification (SDA), Walker, et al., Proc. Natl. Acad. Sci. U.S.A., 89:392 (1992)]; and 3.) methods of library construction and screening by complementation.

For example, genes encoding similar proteins or polypeptides to the threonine deaminase encoding genes provided herein could be isolated directly by using all or a portion of the instant nucleic acid fragments as DNA hybridization probes to screen libraries from any desired organism using methodology well known to those skilled in the art. Specific oligonucleotide probes based upon the disclosed nucleic acid sequences can be designed and synthesized by methods known in the art (Maniatis, supra). Moreover, the entire sequences can be used directly to synthesize DNA probes by methods known to the skilled artisan (e.g., random primers DNA labeling, nick translation or end-labeling techniques), or RNA probes using available in vitro transcription systems. In addition, specific primers can be designed and used to amplify a part of (or full-length of) the instant sequences. The resulting amplification products can be labeled directly during amplification reactions or labeled after amplification reactions, and used as probes to isolate full-length DNA fragments by hybridization under conditions of appropriate stringency.

Typically, in PCR-type amplification techniques, the primers have different sequences and are not complementary to each other. Depending on the desired test conditions, the sequences of the primers should be designed to provide for both efficient and faithful replication of the target nucleic acid. Methods of PCR primer design are common and well known in the art (Thein and Wallace, “The use of oligonucleotides as specific hybridization probes in the Diagnosis of Genetic Disorders”, in Human Genetic Diseases: A Practical Approach, K. E. Davis Ed., (1986) pp 33-50, IRL: Herndon, V A; and Rychlik, W., In Methods in Molecular Biology, White, B. A. Ed., (1993) Vol. 15, pp 31-39, PCR Protocols: Current Methods and Applications. Humania: Totowa, N.J.).

Generally two short segments of the described sequences may be used in polymerase chain reaction protocols to amplify longer nucleic acid fragments encoding homologous genes from DNA or RNA. The polymerase chain reaction may also be performed on a library of cloned nucleic acid fragments wherein the sequence of one primer is derived from the described nucleic acid fragments, and the sequence of the other primer takes advantage of the presence of the polyadenylic acid tracts to the 3′ end of the mRNA precursor encoding microbial genes.

Alternatively, the second primer sequence may be based upon sequences derived from the cloning vector. For example, the skilled artisan can follow the RACE protocol (Frohman et al., PNAS USA 85:8998 (1988)) to generate cDNAs by using PCR to amplify copies of the region between a single point in the transcript and the 3′ or 5′ end. Primers oriented in the 3′ and 5′ directions can be designed from the instant sequences. Using commercially available 3′ RACE or 5′ RACE systems (e.g., BRL, Gaithersburg, Md.), specific 3′ or 5′ cDNA fragments can be isolated (Ohara et al., PNAS USA 86:5673 (1989); Loh et al., Science 243:217 (1989)).

Alternatively, the provided threonine deaminase encoding sequences may be employed as hybridization reagents for the identification of homologs. The basic components of a nucleic acid hybridization test include a probe, a sample suspected of containing the gene or gene fragment of interest, and a specific hybridization method. Probes are typically single-stranded nucleic acid sequences that are complementary to the nucleic acid sequences to be detected. Probes are “hybridizable” to the nucleic acid sequence to be detected. The probe length can vary from 5 bases to tens of thousands of bases, and will depend upon the specific test to be done. Typically a probe length of about 15 bases to about 30 bases is suitable. Only part of the probe molecule need be complementary to the nucleic acid sequence to be detected. In addition, the complementarity between the probe and the target sequence need not be perfect. Hybridization does occur between imperfectly complementary molecules with the result that a certain fraction of the bases in the hybridized region are not paired with the proper complementary base.

Hybridization methods are well defined. Typically the probe and sample must be mixed under conditions that will permit nucleic acid hybridization. This involves contacting the probe and sample in the presence of an inorganic or organic salt under the proper concentration and temperature conditions. The probe and sample nucleic acids must be in contact for a long enough time that any possible hybridization between the probe and sample nucleic acid may occur. The concentration of probe or target in the mixture will determine the time necessary for hybridization to occur. The higher the probe or target concentration, the shorter the hybridization incubation time needed. Optionally, a chaotropic agent may be added. The chaotropic agent stabilizes nucleic acids by inhibiting nuclease activity. Furthermore, the chaotropic agent allows sensitive and stringent hybridization of short oligonucleotide probes at room temperature (Van Ness and Chen, Nucl. Acids Res. 19:5143-5151 (1991)). Suitable chaotropic agents include guanidinium chloride, guanidinium thiocyanate, sodium thiocyanate, lithium tetrachloroacetate, sodium perchlorate, rubidium tetrachloroacetate, potassium iodide and cesium trifluoroacetate, among others. Typically, the chaotropic agent will be present at a final concentration of about 3 M. If desired, one can add formamide to the hybridization mixture, typically 30-50% (v/v).

Various hybridization solutions can be employed. Typically, these comprise from about 20 to 60% volume, preferably 30%, of a polar organic solvent. A common hybridization solution employs about 30-50% v/v formamide, about 0.15 to 1 M sodium chloride, about 0.05 to 0.1 M buffers (e.g., sodium citrate, Tris-HCl, PIPES or HEPES (pH range about 6-9)), about 0.05 to 0.2% detergent (e.g., sodium dodecylsulfate), or between 0.5-20 mM EDTA, FICOLL (Pharmacia Inc.) (about 300-500 kdal), polyvinylpyrrolidone (about 250-500 kdal) and serum albumin. Also included in the typical hybridization solution will be unlabeled carrier nucleic acids from about 0.1 to 5 mg/mL, fragmented nucleic DNA (e.g., calf thymus or salmon sperm DNA, or yeast RNA), and optionally from about 0.5 to 2% wt/vol glycine. Other additives may also be included, such as volume exclusion agents that include a variety of polar water-soluble or swellable agents (e.g., polyethylene glycol), anionic polymers (e.g., polyacrylate or polymethylacrylate) and anionic saccharidic polymers (e.g., dextran sulfate).

Nucleic acid hybridization is adaptable to a variety of assay formats. One of the most suitable is the sandwich assay format. The sandwich assay is particularly adaptable to hybridization under non-denaturing conditions. A primary component of a sandwich-type assay is a solid support. The solid support has adsorbed to it or covalently coupled to it immobilized nucleic acid probe that is unlabeled and complementary to one portion of the sequence.

Threonine deaminase and isopropylmalate synthase, and optionally branched chain amino acid aminotransferase and/or pyruvate dehydrogenase activities may be reduced using genetic manipulations that disrupt expression of active enzyme from the target gene. Many methods for genetic modification of target genes are known to one skilled in the art and may be used to create the present yeast strains. Modifications that may be used to reduce or eliminate expression of a target protein are disruptions that include, but are not limited to, deletion of the entire gene or a portion of the gene, inserting a DNA fragment into the gene (in either the promoter or coding region) so that the protein is not expressed or expressed at lower levels, introducing a mutation into the coding region which adds a stop codon or frame shift such that a functional protein is not expressed, and introducing one or more mutations into the coding region to alter amino acids so that a non-functional or a less enzymatically active protein is expressed. In addition, expression of a gene may be blocked by expression of an antisense RNA or an interfering RNA, and constructs may be introduced that result in cosuppression. In addition, the synthesis or stability of the transcript may be lessened by mutation. Similarly the efficiency by which a protein is translated from mRNA may be modulated by mutation. In addition, since the target proteins are all mitochondrial, disruption of mitochondrial localization may be used such as disrupting the mitochondrial targeting signal sequence. All of these methods may be readily practiced by one skilled in the art making use of the known or identified coding sequences as exemplified in Table 1.

DNA sequences surrounding a target gene coding sequence are also useful in some modification procedures and are available for yeasts such as for Saccharomyces cerevisiae in the complete genome sequence coordinated by Genome Project ID9518 of Genome Projects coordinated by NCBI (National Center for Biotechnology Information) with identifying GOPID #13838. Additional examples of yeast genomic sequences include that of Yarrowia lipolytica, GOPIC #13837, and of Candida albicans, which is included in GPID #10771, #10701 and #16373. Additional genomes have been completely sequenced and annotated and are publicly available for the following yeast strains Candida glabrata CBS 138, Kluyveromyces lactis NRRL Y-1140, Pichia stipitis CBS 6054, and Schizosaccharomyces pombe 972h-.

In particular, DNA sequences surrounding a target coding sequence are useful for modification methods using homologous recombination. For example, in this method flanking sequences are placed bounding a selectable marker gene to mediate homologous recombination whereby the marker gene replaces the target gene. Also partial target gene sequences and flanking sequences bounding a selectable marker gene may be used to mediate homologous recombination whereby the marker gene replaces a portion of the target gene. In addition, the selectable marker may be bounded by site-specific recombination sites, so that following expression of the corresponding site-specific recombinase, the resistance gene is excised from the target gene without reactivating the latter. The site-specific recombination leaves behind a recombination site which disrupts expression of the target gene encoded protein. The homologous recombination vector may be constructed to also leave a deletion in the target gene following excision of the selectable marker, as is well known to one skilled in the art.

Deletions may be made using mitotic recombination as described in Wach et al. ((1994) Yeast 10:1793-1808). This method involves preparing a DNA fragment that contains a selectable marker between genomic regions that may be as short as 20 bp, and which bound a target DNA sequence. This DNA fragment can be prepared by PCR amplification of the selectable marker gene using as primers oligonucleotides that hybridize to the ends of the marker gene and that include the genomic regions that can recombine with the yeast genome. The linear DNA fragment can be efficiently transformed into yeast and recombined into the genome resulting in gene replacement including with deletion of the target DNA sequence (as described in Methods in Enzymology, v194, pp 281-301 (1991)).

Moreover, promoter replacement methods may be used to exchange the endogenous transcriptional control elements allowing another means to modulate expression such as described in Mnaimneh et al. ((2004) Cell 118(1):31-44) and in Example 12 herein.

In addition, a target gene in any yeast cell may be disrupted using random mutagenesis, which is followed by screening to identify strains with reduced target gene encided activity. Using this type of method, the DNA sequence of any region of the genome affecting expression of a target protein need not be known. Methods for creating genetic mutations are common and well known in the art and may be applied to the exercise of creating mutants. Commonly used random genetic modification methods (reviewed in Methods in Yeast Genetics, 2005, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.) include spontaneous mutagenesis, mutagenesis caused by mutator genes, chemical mutagenesis, irradiation with UV or X-rays, or transposon mutagenesis.

Chemical mutagenesis of yeast commonly involves treatment of yeast cells with one of the following DNA mutagens: ethyl methanesulfonate (EMS), nitrous acid, diethyl sulfate, or N-methyl-N′-nitro-N-nitroso-guanidine (MNNG). These methods of mutagenesis have been reviewed in Spencer et al (Mutagenesis in Yeast, 1996, Yeast Protocols: Methods in Cell and Molecular Biology. Humana Press, Totowa, N.J.). Chemical mutagenesis with EMS may be performed as described in Methods in Yeast Genetics, 2005, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. Irradiation with ultraviolet (UV) light or X-rays can also be used to produce random mutagenesis in yeast cells. The primary effect of mutagenesis by UV irradiation is the formation of pyrimidine dimers which disrupt the fidelity of DNA replication. Protocols for UV-mutagenesis of yeast can be found in Spencer et al (Mutagenesis in Yeast, 1996, Yeast Protocols: Methods in Cell and Molecular Biology. Humana Press, Totowa, N.J.). Introduction of a mutator phenotype can also be used to generate random chromosomal mutations in yeast. Common mutator phenotypes can be obtained through disruption of one or more of the following genes: PMS1, MAG1, RAD18 or RAD51. Restoration of the non-mutator phenotype can be easily obtained by insertion of the wildtype allele. Collections of modified cells produced from any of these or other known random mutagenesis processes may be screened for reduced activity of the target enzyme.

Production of Isobutanol

Strains of yeast disclosed herein may be grown in fermentation media for production of isobutanol. Suitable carbon substrates may include but are not limited to monosaccharides such as fructose, oligosaccharides such as lactose maltose, galactose, or sucrose, polysaccharides such as starch or cellulose or mixtures thereof and unpurified mixtures from renewable feedstocks such as cheese whey permeate, cornsteep liquor, sugar beet molasses, and barley malt. Other carbon substrates may include ethanol, lactate, succinate, or glycerol.

Additionally the carbon substrate may also be one-carbon substrates such as carbon dioxide, or methanol for which metabolic conversion into key biochemical intermediates has been demonstrated. In addition to one and two carbon substrates, methylotrophic organisms are also known to utilize a number of other carbon containing compounds such as methylamine, glucosamine and a variety of amino acids for metabolic activity. For example, methylotrophic yeasts are known to utilize the carbon from methylamine to form trehalose or glycerol (Bellion et al., Microb. Growth C1 Compd., [Int. Symp.], 7th (1993), 415-32, Editor(s): Murrell, J. Collin; Kelly, Don P. Publisher: Intercept, Andover, UK). Similarly, various species of Candida will metabolize alanine or oleic acid (Sulter et al., Arch. Microbiol. 153:485-489 (1990)). Hence it is contemplated that the source of carbon utilized in the present invention may encompass a wide variety of carbon containing substrates.

Although it is contemplated that all of the above mentioned carbon substrates and mixtures thereof are suitable in the present invention, preferred carbon substrates are glucose, fructose, and sucrose, or mixtures of these with C5 sugars such as xylose and/or arabinose for yeasts cells modified to use C5 sugars. Sucrose may be derived from renewable sugar sources such as sugar cane, sugar beets, cassava, sweet sorghum, and mixtures thereof. Glucose and dextrose may be derived from renewable grain sources through saccharification of starch based feedstocks including grains such as corn, wheat, rye, barley, oats, and mixtures thereof. In addition, fermentable sugars may be derived from renewable cellulosic or lignocellulosic biomass through processes of pretreatment and saccharification, as described, for example, in U.S. Patent Application Publication No. 2007/0031918A1, which is herein incorporated by reference. Biomass refers to any cellulosic or lignocellulosic material and includes materials comprising cellulose, and optionally further comprising hemicellulose, lignin, starch, oligosaccharides and/or monosaccharides. Biomass may also comprise additional components, such as protein and/or lipid. Biomass may be derived from a single source, or biomass can comprise a mixture derived from more than one source; for example, biomass may comprise a mixture of corn cobs and corn stover, or a mixture of grass and leaves. Biomass includes, but is not limited to, bioenergy crops, agricultural residues, municipal solid waste, industrial solid waste, sludge from paper manufacture, yard waste, wood and forestry waste. Examples of biomass include, but are not limited to, corn grain, corn cobs, crop residues such as corn husks, corn stover, grasses, wheat, wheat straw, barley, barley straw, hay, rice straw, switchgrass, waste paper, sugar cane bagasse, sorghum, soy, components obtained from milling of grains, trees, branches, roots, leaves, wood chips, sawdust, shrubs and bushes, vegetables, fruits, flowers, animal manure, and mixtures thereof.

In addition to an appropriate carbon source, fermentation media must contain suitable minerals, salts, cofactors, buffers and other components, known to those skilled in the art, suitable for the growth of the cultures and promotion of the enzymatic pathway necessary for production of the desired product. Byproduct formation

It will be appreciated that reduction and preferably elimination of by-products of carbon metabolism other than carbon dioxide and isobutanol would be advantageous for production of isobutanol. For example microorganisms metabolizing sugar substrates produce a variety of by-products in a mixed acid fermentation (Moat, A. G. et al., MicrobialPhysiology, 4th edition, John Wiley Publishers, N.Y., 2002). Yeast metabolizing sugar substrates produce a variety of by-products like acids and alcohols such as, but not limited to, formate, lactate, succinate, ethanol, acetate and glycerol. Formation of these byproducts during isobutanol fermentation lower the yield of isobutanol. To prevent yield loss of isobutanol the genes encoding enzyme activities corresponding to byproduct formation can be down-regulated or disrupted using methods described herein and/or known in the art.

Endogenous pyruvate decarboxylase activity in yeast converts pyruvate to acetaldehyde, which is then converted to ethanol or to acetyl-CoA via acetate. Therefore, endogenous pyruvate decarboxylase activity is a target for reduction of byproduct formation. Yeasts may have one or more genes encoding pyruvate decarboylase. For example, there is one gene encoding pyruvate decarboxylase in Kluyveromyces lactis, while there are three isozymes of pyruvate decarboxylase encoded by the PDC1, PCD5, and PDC6 genes in Saccharomyces cerevisiae, as well as a pyruvate decarboxylase regulatory gene PDC2. Expression of pyruvate decarboxylase from PDC6 is minimal. In yeast strains disclosed herein, the pyruvate decarboxylase activity may be reduced by downregulating or disrupting at least one gene encoding a pyruvate decarboxylase, or a gene regulating pyruvate decarboxylase gene expression as described in U.S. patent application Ser. No. 12/477,942, which is herein incorporated by reference. For example, in S. cerevisiae the PDC1 and PDC5 genes, or all three genes, may be disrupted. Alternatively, pyruvate decarboxylase activity may be reduced by disrupting the PDC2 regulatory gene in S. cerevisiae. In other yeasts, genes encoding pyruvate decarboxylase proteins such as those having at least about 80-85%, 85%-90%, 90%-95%, or at least about 98% sequence identity to PDC1 or PDC5 may be downregulated or disrupted. Examples of yeast pyruvate decarboxylase genes or proteins that may be targeted for downregulation or disruption are listed in Table 3 (SEQ ID NOs: 108, 110, 112, 114, 116, 118, 120, 122, and 124).

Examples of yeast strains with reduced pyruvate decarboxylase activity due to disruption of pyruvate decarboxylase encoding genes have been reported such as for Saccharomyces in Flikweert et al. (Yeast (1996) 12:247-257), for Kluyveromyces in Bianchi et al. (Mol. Microbiol. (1996) 19(1):27-36), and disruption of the regulatory gene in Hohmann, (Mol Gen Genet. (1993) 241:657-666). Saccharomyces strains having no pyruvate decarboxylase activity are available from the ATCC (Accession #200027 and #200028).

Culture Conditions

Typically cells are grown at a temperature in the range of about 20° C. to about 37° C. in an appropriate medium. Suitable growth media in the present invention are common commercially prepared media such as broth that includes yeast nitrogen base, ammonium sulfate, and dextrose as the carbon/energy source) or YPD Medium, a blend of peptone, yeast extract, and dextrose in optimal proportions for growing most Saccharomyces cerevisiae strains. Other defined or synthetic growth media may also be used and the appropriate medium for growth of the particular microorganism will be known by one skilled in the art of microbiology or fermentation science.

Suitable pH ranges for the fermentation are between pH 3.0 to pH 7.5, where pH 4.5.0 to pH 6.5 is preferred as the initial condition.

Fermentations may be performed under aerobic or anaerobic conditions, where anaerobic or microaerobic conditions are preferred.

The amount of isobutanol produced in the fermentation medium can be determined using a number of methods known in the art, for example, high performance liquid chromatography (HPLC) or gas chromatography (GC).

Industrial Batch and Continuous Fermentations

The present process employs a batch method of fermentation. A classical batch fermentation is a closed system where the composition of the medium is set at the beginning of the fermentation and not subject to artificial alterations during the fermentation. Thus, at the beginning of the fermentation the medium is inoculated with the desired organism or organisms, and fermentation is permitted to occur without adding anything to the system. Typically, however, a “batch” fermentation is batch with respect to the addition of carbon source and attempts are often made at controlling factors such as pH and oxygen concentration. In batch systems the metabolite and biomass compositions of the system change constantly up to the time the fermentation is stopped. Within batch cultures cells moderate through a static lag phase to a high growth log phase and finally to a stationary phase where growth rate is diminished or halted. If untreated, cells in the stationary phase will eventually die. Cells in log phase generally are responsible for the bulk of production of end product or intermediate.

A variation on the standard batch system is the Fed-Batch system. Fed-Batch fermentation processes are also suitable in the present invention and comprise a typical batch system with the exception that the substrate is added in increments as the fermentation progresses. Fed-Batch systems are useful when catabolite repression is apt to inhibit the metabolism of the cells and where it is desirable to have limited amounts of substrate in the media. Measurement of the actual substrate concentration in Fed-Batch systems is difficult and is therefore estimated on the basis of the changes of measurable factors such as pH, dissolved oxygen and the partial pressure of waste gases such as CO₂. Batch and Fed-Batch fermentations are common and well known in the art and examples may be found in Thomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology, Second Edition (1989) Sinauer Associates, Inc., Sunderland, Mass., or Deshpande, Mukund V., Appl. Biochem. Biotechnol., 36:227, (1992), herein incorporated by reference.

Although the present invention is performed in batch mode it is contemplated that the method would be adaptable to continuous fermentation methods. Continuous fermentation is an open system where a defined fermentation medium is added continuously to a bioreactor and an equal amount of conditioned media is removed simultaneously for processing. Continuous fermentation generally maintains the cultures at a constant high density where cells are primarily in log phase growth.

Continuous fermentation allows for the modulation of one factor or any number of factors that affect cell growth or end product concentration. For example, one method will maintain a limiting nutrient such as the carbon source or nitrogen level at a fixed rate and allow all other parameters to moderate. In other systems a number of factors affecting growth can be altered continuously while the cell concentration, measured by media turbidity, is kept constant. Continuous systems strive to maintain steady state growth conditions and thus the cell loss due to the medium being drawn off must be balanced against the cell growth rate in the fermentation. Methods of modulating nutrients and growth factors for continuous fermentation processes as well as techniques for maximizing the rate of product formation are well known in the art of industrial microbiology and a variety of methods are detailed by Brock, supra.

It is contemplated that the present invention may be practiced using either batch, fed-batch or continuous processes and that any known mode of fermentation would be suitable. Additionally, it is contemplated that cells may be immobilized on a substrate as whole cell catalysts and subjected to fermentation conditions for isobutanol production.

Methods for Isobutanol Isolation from the Fermentation Medium

Bioproduced isobutanol may be isolated from the fermentation medium using methods known in the art. For example, solids may be removed from the fermentation medium by centrifugation, filtration, decantation, or the like. Then, the isobutanol may be isolated from the fermentation medium, which has been treated to remove solids as described above, using methods such as distillation, liquid-liquid extraction, or membrane-based separation. Because isobutanol forms a low boiling point, azeotropic mixture with water, distillation can only be used to separate the mixture up to its azeotropic composition. Distillation may be used in combination with another separation method to obtain separation around the azeotrope. Methods that may be used in combination with distillation to isolate and purify isobutanol include, but are not limited to, decantation, liquid-liquid extraction, adsorption, and membrane-based techniques. Additionally, isobutanol may be isolated using azeotropic distillation using an entrainer (see for example Doherty and Malone, Conceptual Design of Distillation Systems, McGraw Hill, N.Y., 2001).

The isobutanol-water mixture forms a heterogeneous azeotrope so that distillation may be used in combination with decantation to isolate and purify the isobutanol. In this method, the isobutanol containing fermentation broth is distilled to near the azeotropic composition. Then, the azeotropic mixture is condensed, and the isobutanol is separated from the fermentation medium by decantation. The decanted aqueous phase may be returned to the first distillation column as reflux. The isobutanol-rich decanted organic phase may be further purified by distillation in a second distillation column.

The isobutanol may also be isolated from the fermentation medium using liquid-liquid extraction in combination with distillation. In this method, the isobutanol is extracted from the fermentation broth using liquid-liquid extraction with a suitable solvent. The isobutanol-containing organic phase is then distilled to separate the isobutanol from the solvent.

Distillation in combination with adsorption may also be used to isolate isobutanol from the fermentation medium. In this method, the fermentation broth containing the isobutanol is distilled to near the azeotropic composition and then the remaining water is removed by use of an adsorbent, such as molecular sieves (Aden et al. Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover, Report NREL/TP-510-32438, National Renewable Energy Laboratory, June 2002).

Additionally, distillation in combination with pervaporation may be used to isolate and purify the isobutanol from the fermentation medium. In this method, the fermentation broth containing the isobutanol is distilled to near the azeotropic composition, and then the remaining water is removed by pervaporation through a hydrophilic membrane (Guo et al., J. Membr. Sci. 245, 199-210 (2004)).

EXAMPLES

The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various uses and conditions.

General Methods

Standard recombinant DNA and molecular cloning techniques used in the Examples are well known in the art and are described by Sambrook, J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory Press: Cold Spring Harbor, N.Y. (1989) (Maniatis) and by T. J. Silhavy, M. L. Bennan, and L. W. Enquist, Experiments with Gene Fusions, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1984) and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, pub. by Greene Publishing Assoc. and Wiley-Interscience (1987), and by Methods in Yeast Genetics, 2005, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.

Materials and methods suitable for the maintenance and growth of bacterial cultures are well known in the art. Techniques suitable for use in the following Examples may be found as set out in Manual of Methods for General Bacteriology (Phillipp Gerhardt, R. G. E. Murray, Ralph N. Costilow, Eugene W. Nester, Willis A. Wood, Noel R. Krieg and G. Briggs Phillips, eds), American Society for Microbiology, Washington, D.C. (1994)) or by Thomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology, Second Edition, Sinauer Associates, Inc., Sunderland, Mass. (1989). All reagents, restriction enzymes and materials used for the growth and maintenance of microbial cells were obtained from Aldrich Chemicals (Milwaukee, Wis.), BD Diagnostic Systems (Sparks, Md.), Life Technologies (Rockville, Md.), or Sigma Chemical Company (St. Louis, Mo.) unless otherwise specified. Microbial strains were obtained from The American Type Culture Collection (ATCC), Manassas, Va., unless otherwise noted. The oligonucleotide primers used in the following Examples are given in Table 2. All the oligonucleotide primers were synthesized by Sigma-Genosys (Woodlands, Tex.) or Integrated DNA Technologies (Coralsville, Iowa).

Synthetic complete medium is described in Amberg, Burke and Strathern, 2005, Methods in Yeast Genetics, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.

GC Method

The GC method utilized an HP-InnoWax column (30 m×0.32 mm ID, 0.25 μm film) from Agilent Technologies (Santa Clara, Calif.). The carrier gas was helium at a flow rate of 1 ml/min measured at 150° C. with constant head pressure; injector split was 1:10 at 200° C.; oven temperature was 45° C. for 1 min, 45° C. to 230° C. at 10° C./min, and 230° C. for 30 sec. FID detection was used at 260° C. with 40 ml/min helium makeup gas. Culture broth samples were filtered through 0.2 μM spin filters before injection. Depending on analytical sensitivity desired, either 0.1 μl or 0.5 μl injection volumes were used. Calibrated standard curves were generated for the following compounds: ethanol, isobutanol, acetoin, meso-2,3-butanediol, and (2S,3S)-2,3-butanediol. Analytical standards were also utilized to identify retention times for isobutyraldehyde, isobutyric acid, and isoamyl alcohol.

SEQ ID NOs for primers and vectors in the examples below are listed in Table 2.

The meaning of abbreviations is as follows: “s” means second(s), “min” means minute(s), “h” means hour(s), “psi” means pounds per square inch, “nm” means nanometers, “d” means day(s), “μL” means microliter(s), “mL” means milliliter(s), “L” means liter(s), “mm” means millimeter(s), “nm” means nanometers, “mM” means millimolar, “M” means molar, “mmol” means millimole(s), “μmol” means micromole(s)“, “g” means gram(s), “μg” means microgram(s) and “ng” means nanogram(s), “PCR” means polymerase chain reaction, “OD” means optical density, “OD₆₀₀” means the optical density measured at a wavelength of 600 nm, “kDa” means kilodaltons, “g” means the gravitation constant, “bp” means base pair(s), “kbp” means kilobase pair(s), “% w/v” means weight/volume percent, % w/v” means volume/volume percent, “wt %” means percent by weight, “HPLC” means high performance liquid chromatography, “GC” means gas chromatography, “FID” means flame ionization detector.

Example 1 Eliminating Expression of Enzymes in Branched Chain Amino Acid Biosynthesis Pathways in S. cerevisiae

Yeast cells were engineered to eliminate activities of specific enzymes of mitochondrial branched chain amino acid biosynthesis. Three chromosomal disruptions were generated in successive fashion in the following genes: ILV1, encoding threonine deaminase; LEU4, encoding 2-isopropylmalate synthase; and BAT1, encoding branched chain amino acid aminotransferase.

An ilv1::LEU2 cassette was constructed by PCR amplification of the LEU2 marker from pRS425 (ATCC No. 77106) using Phusion DNA polymerase (New England Biolabs Inc., Beverly, Mass.; catalog no. F-540S) with primers 112590-88A (SEQ ID NO:81) and 112590-88B (SEQ ID NO:82). The ILV1 portion of each primer was derived from the 5′ region upstream of the ILV1 promoter and 3′ region downstream of the transcriptional terminator, respectively, such that integration of the LEU2 marker results in replacement of the ILV1 coding region. The ˜1.7 kb PCR product was transformed into Saccharomyces cerevisiae strain BY4741 (ATCC #201388) with selection on synthetic complete media lacking leucine and supplemented with 2% glucose at 30° C. Transformants were screened by colony PCR using primers 112590-88C (SEQ ID NO:83) and 112590-88D (SEQ ID NO:84). The resulting identified strain had the genotype: BY4741 ilv1::LEU2.

A leu4::URA3r disruption cassette was constructed by PCR amplification of the URA3r marker from pUC19-URA3r (SEQ ID NO:85) using Phusion DNA polymerase (New England Biolabs Inc., Beverly, Mass.; catalog no. F-540S) and primers 112590-97A (SEQ ID NO:86) and 112590-97B (SEQ ID NO:87). pUC19-URA3r contains the URA3 marker from pRS426 (ATCC no. 77107) flanked by 75 bp homologous repeat sequences to allow homologous recombination in vivo and removal of the URA3 marker. The LEU4 portion of each primer was derived from the 5′ region upstream of the LEU4 promoter and 3′region downstream of the transcriptional terminator, respectively, such that integration of the URA3r marker results in replacement of the LEU4 coding region. The ˜1.5 kb PCR product was transformed into BY4741 ilv1::LEU2 cells with selection on synthetic complete media lacking uracil and supplemented with 2% glucose at 30° C. Transformants were screened by PCR using external primers 112590-49E (SEQ ID NO:88) and 112590-97C (SEQ ID NO:89) to verify integration at the correct site. The URA3r marker was recycled by plating on synthetic complete media supplemented with 2% glucose and 5-FOA at 30° C. following standard protocols. Marker removal was confirmed by patching colonies from the 5-FOA plates onto synthetic complete media lacking uracil to verify the absence of growth. The resulting identified strain had the genotype: BY4741 Δilv1Δleu4.

A bat1::URA3r disruption cassette was constructed in several steps. A cassette containing the BAT1 5′ region was amplified from BY4741 genomic DNA using Phusion DNA polymerase (New England Biolabs Inc., Beverly, Mass.; catalog no. F-540S) and primers 112590-108A (SEQ ID NO:90) and 112590-108B (SEQ ID NO:91). The cassette containing BAT1 3′sequences was amplified from BY4741 genomic DNA using Phusion DNA polymerase and primers 112590-108C (SEQ ID NO:92) and 112590-108D (SEQ ID NO:93). The URA3r marker was PCR-amplified from pUC19-URA3r using Phusion DNA polymerase and primers 112590-108E (SEQ ID NO:94) and 112590-108F (SEQ ID NO:95). The three PCR products were combined in a SOE PCR reaction (Horton et al. (1989) Gene 77:61-68) and amplified using Phusion DNA polymerase and the end primers 112590-108A (SEQ ID NO:90) and 112590-108D (SEQ ID NO:93), generating the full ˜2.8 kb BAT1::URA3r disruption cassette. The BAT1 portion of each primer was derived from the 5′ region upstream of the BAT1 promoter and 3′ region downstream of the transcriptional terminator, respectively, such that integration of the URA3r marker results in replacement of the BAT1 coding region. The cassette was transformed into BY4741 Δilv1 Δleu4 with selection on synthetic complete media lacking uracil and supplemented with 2% glucose at 30° C. Transformants were screened by PCR using external primers 112590-49E (SEQ ID NO:88) and “BAT1 check” (SEQ ID NO:96) to verify integration at the correct site. The URA3r marker was recycled by plating on synthetic complete media supplemented with 2% glucose and 5-FOA at 30° C. following standard protocols. Marker removal was confirmed by patching colonies from the 5-FOA plates onto synthetic complete media lacking uracil supplemented with 2% glucose to verify the absence of growth. The resulting identified strain had the genotype: BY4741 Δilv1 Δleu4 Δbat1.

Example 2 Reduction of Pyruvate Dehydrogenase Activity in S. cerevisiae

To reduce levels of mitochondrial pyruvate dehydrogenase activity, the native promoter of the PDA1 gene, encoding the E1α subunit of pyruvate dehydrogenase, was replaced with the inducible GAL1 promoter through homologous recombination. The GAL1 promoter and URA3r marker were joined together by SOE PCR. The URA3r marker was PCR amplified from pUC19-URA3r using Phusion DNA polymerase (New England Biolabs Inc., Beverly, Mass. alog no. F-540S) and primers 112590-118A (SEQ ID NO:97) and 112590-118B (SEQ ID NO:98). The GAL1 promoter was PCR-amplified from pRS426::GAL1p-alsS (SEQ ID NO:99) using Phusion DNA polymerase (New England Biolabs Inc., Beverly, Mass. catalog no. F-540S) and primers 112590-118C (SEQ ID NO:100) and 112590-118D (SEQ ID NO:101). pRS426::GAL1p-alsS contained an F1 origin of replication (nt 4976 to 5432) for maintenance in E. coli and a 2 micron origin (nt 2215 to 3560) for replication in yeast. The vector has an GAL1 promoter (nt 7702 to 8144) and CYC1 terminator (nt 5721 to 5970). In addition, it carries the URA3 marker (nt 4042 to 4845) for selection in yeast and ampicillin resistance marker (nt 1225 to 2082) for selection in E. coli.

The two products were joined by SOE PCR using Phusion DNA polymerase and primers 112590-118E (SEQ ID NO:102) and 112590-118F (SEQ ID NO:103). The PDA1 portion of each primer was derived from the 5′ region upstream of the PDA1 promoter and PDA1 coding sequence, respectively, such that integration of the URA3 marker results in replacement of the native PDA1 promoter with the GAL1 promoter. The PCR product was transformed into BY4741 Δilv1 Δleu4 Δbat1 with selection on synthetic complete media lacking uracil and supplemented with 2% glucose at 30° C. Transformants were screened by PCR using external primers 112590-49E (SEQ ID NO:88) and 112590-118G (SEQ ID NO:104) to verify integration at the PDA1 locus. The URA3r marker was recycled by plating on synthetic complete media supplemented with 2% glucose and 5-FOA at 30° C. following standard protocols. Marker removal was confirmed by patching colonies from the 5FOA plates onto synthetic complete media lacking uracil supplemented with 2% glucose to verify the absence of growth. The resulting identified strain had the genotype: BY4741 Δilv1 Δleu4 Δbat1 GAL1p-PDA1.

Example 3 Isobutanol Production in S. cerevisiae Deletion Strains

The purpose of this example is to demonstrate isobutanol production in S. cerevisiae in which ILV1 (threonine deaminase) and LEU4 (2-isopropylmalate synthase), and optionally BAT1 (branched-chain amino acid aminotransferase) genes are disrupted. An additional strain has a PDA1 (pyruvate dehydrogenase) disruption.

Strains BY4741, BY4741 Δilv1 Δleu4, BY4741 Δilv1 Δleu4 Δbat1, and BY4741 Δilv1 Δleu4 Δbat1 GAL1p-PDA1, each transformed with control pR423 (ATCC #77104) and pRS426 plasmids, were grown in synthetic complete media lacking histidine and uracil, and supplemented with 2% glucose under aerobic conditions (20 ml media in 125 ml flask) and cultured at 30° C. with shaking at 220 rpm. Cultures were inoculated at 0.1 OD₆₀₀ and assayed for isobutanol titers at 24 hours post-inoculation. Isobutanol was quantitated by GC-FID on a HP-Innowax column using a standard curve of pure isobutanol. A standard curve of isobutanol ranging from 25 mM to 0.6 mM was used to define the linear relationship between raw peak area and isobutanol concentration. Experimental samples were compared against this standard curve to obtain isobutanol titers given in Table 4.

TABLE 4 Isobutanol production in control and different deletion strains of S. cerevisiae. Strain Isobutanol (mM) BY4741 pRS423/pRS426 0.07* BY4741 Δilv1 Δleu4 pRS423/ 0.64 pRS426 BY4741 Δilv1 Δleu4 Δbat1 0.90 pRS423/pRS426 BY4741 Δilv1 Δleu4 Δbat1 0.95 GAL1p-PDA1 pRS423/pRS426 *Sample taken at 48 hours. 

What is claimed is:
 1. A recombinant yeast host cell which produces isobutanol and comprises mitochondria which is substantially devoid of isopropylmalate synthase activity.
 2. The recombinant yeast cell of claim 1, wherein the isopropylmalate synthase activity is defined by the enzyme classification number EC 2.3.3.13.
 3. The recombinant yeast cell of claim 1, wherein the yeast cell comprises a disruption in a LEU4 gene.
 4. The recombinant yeast cell of claim 1, wherein the mitochondria is further substantially devoid of branched chain amino acid transaminase activity.
 5. The recombinant yeast cell of claim 4, wherein the branched chain amino acid transaminase activity is defined by the enzyme classification number EC 2.6.1.42.
 6. The recombinant yeast cell of claim 4, wherein the yeast cell comprises a disruption in a BAT1 gene.
 7. The recombinant yeast cell of claim 1, wherein the mitochondria is further substantially devoid of pyruvate dehydrogenase activity.
 8. The recombinant yeast cell of claim 7, wherein the pyruvate dehydrogenase activity is defined by the enzyme classification number EC 1.2.4.1.
 9. The recombinant yeast cell of claim 7, wherein the pyruvate dehydrogenase activity is defined by a multienzyme complex comprising proteins selected from the group consisting of PDA1, PDB1, LAT1, LPD1, and PDX1.
 10. The recombinant yeast cell of claim 9, wherein the recombinant yeast cell comprises a disruption in a gene encoding a protein selected from the group consisting of PDA1, PDB1, LAT1, LPD1, and PDX1.
 11. The recombinant yeast cell of claim 1, wherein the recombinant yeast cell further comprises reduced or eliminated endogenous pyruvate decarboxylase activity.
 12. The recombinant yeast cell of claim 1, wherein the yeast is selected from the group consisting of Saccharomyces, Schizosaccharomyces, Hansenula, Candida, Kluyveromyces, Yarrowia, and Pichia. 